In a power steering apparatus which is mounted on an automobile to alleviate the force required by a driver to steer the automobile, a pump which acts as a source of pressurized oil pressure is normally driven for rotation by an engine which is mounted on the automobile. The discharge of the working oil from the pump increases or decreases in proportion to a change in the number of revolutions of the engine. Accordingly, in a pump of a type which is responsive to the number of revolutions, it is required that the pump have a capacity to be able to supply a sufficient flow to a hydraulic instrument such as a power steering apparatus even when the engine operates in a range of a low number of revolutions or when the pump discharge is low. However, the choice of such capacity for the pump is insufficient by itself in that when the engine operates with a higher number of revolutions, an unnecessarily increased flow will be supplied to the power steering apparatus. In such instance, it is necessary to return an excess amount of flow to a tank.
To accommodate for this, it has been a conventional practice to provide an orifice in an oil pressure supply passage which extends from the pump to the power steering apparatus so that a pressure differential across the orifice operates to open a spool valve, acting as a flow control valve, so that part of the flow or pressurized oil which is in excess of that required to operate the power steering apparatus smoothly returned to the tank without causing a flow resistance, thereby allowing the flow rate being supplied to the power steering apparatus to be maintained at a constant amount or less.
However, in a pressurized oil supply system as may be used for the power steering apparatus described above, the amount of pressurized oil being supplied when the engine of the automobile operates in a range of a high number of revolutions presents a problem in respect of the running stability of the automobile. Specifically, when running the automobile at a high speed, the driver is subject to a level of anxiety if the steering wheel can be maneuvered too lightly. To eliminate such difficulty, a drooping response, namely, a reduction of the amount of pressurized oil being supplied to a certain degree as the number of revolutions increases is useful to achieve a vehicle stability or an adequate sensation of steering when running at a high speed and for the purpose of achieving a saving of power.
To this end, the orifice which has been used in the prior art for purpose of controlling the flow rate is constructed as a variable throttle structure so that it may function as a fixed throttle when the automobile is parked or is running at low or medium speeds, while when the automobile is running at a higher speed, the orifice is throttled down, so that the resulting pressure differential thereacross may be utilized to increase the amount of flow returned to the tank, thus reducing the supply of pressurized oil to the power steering apparatus.
However, when such a flow control including the drooping response is attempted through a simple variable control of the orifice, there occurs a pressure rise in the pressurized oil supply system when the power steering apparatus is operated for steering purpose to change the pressure differential across the orifice, which is reflected as a restriction of the return flow of the excess amount by the spool valve to result in an increase in the flow rate being supplied which has once been reduced in accordance with the magnitude of the pressure rise, thus destroying the very purpose even though a desired flow control can be achieved, including the drooping response, when the power steering apparatus is not operated or is under a no-load condition.
To accommodate for this difficulty, a flow control valve unit has been proposed in which part of the supply passage is formed with a restriction, with a control spool, which is intended to restrict the flow, responding to a pressure differential across the restriction in order to reduce the throttling action of the orifice, as disclosed in Japanese Laid-Open Patent Application No. 4,469/1982 (Japanese Granted Patent Publication No. 37749/1988). Specifically, the orifice is made to be controllable in a variable manner on the basis of an increase in the pump discharge, which is caused by an increase in the number of revolutions of the pump, independently from the displacement of the spool valve, thus preventing the flow rate from returning to its original value upon operation of the power steering apparatus. With this arrangement, the described difficulty caused by a control of the flow rate through the variable control of the simple orifice can be eliminated by the combination of the restriction and the control spool which variably controls the orifice in accordance with the pressure differential produced across the restriction, thus providing an enhanced usefulness.
However, in a flow control valve unit having a non-return drooping response contructed in a manner mentioned above, a control spool must be added in order to enable a variable control of the orifice. As a consequence, in view of its relationship with a flow controlling spool valve, the number of components must be increased, and the construction must be complicated. In addition, in order to achieve a required flow control response, a high demand is imposed in the accuracy of machining the components and in adjusting the timing of operation. It is also difficult to adjust the flow control response including the drooping response independently from the magnitude of an increase in the load pressure as the power steering apparatus is operated, and accordingly, it is desired that some remedy be provided which eliminates all of these difficulties.
To this end, the present applicant has previously proposed a flow control valve which has simplified the construction and reduced the number of parts required while facilitating an adjustment of the drooping response (see Japanese Laid-Open Patent Application No. 132,471/1989). In this flow control valve, a tubular union is fixedly mounted in a connector, which is disposed in a spool valve receiving bore, and the union is formed with orifices, while a sleeve fitted around the union is disposed for sliding movement in response to a pressure differential across a restriction so that the orifices may be controlled or reduced. This valve achieves the described objective, but it is also desirable to provide a flow control valve unit in which the number of parts is further reduced and which is capable of fully functioning if the accuracy of machining is reduced.